Conventionally, an amount of fuel injected to an engine is controlled by controlling a period of injection of the fuel from an injector. In the injector, an electric current is supplied to a solenoid to slide a needle blocking the injection hole of the injector so as to open the injection hole. Thus, the fuel injection period from the injector is decided according to the period during which an electric current is supplied to the solenoid. When this current-supplying period to the solenoid is sufficiently long, the needle will move accurately, and thus, the fuel injection period and, hence, the fuel injection amount can be controlled appropriately. If the current-supplying period to the solenoid is short, the amount of movement of the needle is insufficient, causing unstable movement thereof. This may lead to unstable fuel injection period and, hence, unstable fuel injection amount. Accordingly, a lower limit is provided for the fuel injection period. With provision of such a lower limit for the fuel injection period, however, it is not possible to reduce the fuel injection amount by reducing the fuel injection period beyond the lower limit. As such, there is a technique to reduce the amount of the fuel injected per unit time by lowering the pressure of the fuel (fuel pressure) in the case where the fuel injection period becomes short to reach the lower limit (when the fewer fuel injection amount is required).
Japanese Patent Laying-Open No. 09-021369 discloses a fuel injection control apparatus for an internal combustion engine, which includes a fuel pressurizing portion that pressurizes fuel, a fuel injection portion that injects the fuel pressurized by the fuel pressurizing portion, a fuel injection amount setting portion that sets the amount of the fuel to be injected by the fuel injection portion in accordance with an engine operation state, an injection start timing setting portion that sets the injection start timing with respect to a prescribed injection end timing based on the fuel injection amount set by the fuel injection amount setting portion, and a pressure changing portion that lowers the pressure of the fuel pressurizing portion when the fuel injection amount set by the fuel injection amount setting portion is small.
According to the fuel injection control apparatus described in this publication, the injection start timing setting portion sets the injection start timing with respect to a prescribed injection end timing, based on the injection amount set by the fuel injection amount setting portion in accordance with the engine operation state. When the fuel injection amount set by the fuel injection amount setting portion is small, the pressure changing portion controls to lower the pressurizing force of the fuel pressurizing portion, and thus, the fuel pressure is lowered. As such, it is possible to reduce the amount of the fuel injected per unit time.
When the pressure of the fuel pressurizing portion, i.e., the fuel pressure, is lowered as in the fuel injection control apparatus described in Japanese Patent Laying-Open No. 09-021369, however, atomization of the fuel would be degraded (the fuel injected in the atomized state would decrease). With degradation in atomization of the fuel, the combustion state of the air-fuel mixture would change, leading to large variation in output of the engine.